The present invention relates to sodium vapor lamps and in particular to an improved method for fabricating such lamps and improved lamps resulting therefrom.
High pressure sodium vapor lamps, which constitute one known type of saturated vapor pressure electric discharge lamps, are known in the art. Those which are commercially available are filled with large amounts of sodium and mercury, and parts thereof become accumulated in the liquid phase as an amalgam at the coolest points within the arc tube of the lamp. In such lamps, the operating characteristics, especially the arc voltage, tend to fluctuate depending on those factors which will affect the temperature of the coolest point in the arc tube. One factor, for example, would be a variation of the source voltage.
It is theoretically known that the above shortcoming can be overcome by limiting the amount of sodium and mercury to that which will become totally vaporized. In this connection it is possible to work out by calculation, the maximum allowable amount of the sodium and mercury, i.e. the maximum amount which would not result in the condensation of the substances under working condition.
Despite the latter, no one has previously been able to discover and disclose the appropriate amount of fillers (i.e. sodium and mercury) to be used for a practical electric discharge lamp having high efficiency, acceptable color rendition, long service life and suitable stabilized arc voltage using an economical ballast. This is attributable to the fact that the appropriate amount of fillers is extremely small and that sodium is one of the chemically active elements. It is quite difficult to pick up and accurately fill such a small amount of chemically active sodium without being contaminated by other atmospheric elements such as oxygen or moisture. At the same time, it is very difficult to form a very small drop of mercury on an industrial scale, since mercury has a large surface tension and a large specific gravity.